The present invention relates to a control circuit for providing low dropout voltage regulation in a series voltage regulator circuit. More particularly, the present invention relates to a three terminal control circuit for driving a discrete PNP transistor or p-channel FET to provide a low dropout positive series voltage regulator circuit.
A series voltage regulator circuit requires a minimum voltage differential between the supply voltage and the regulated output voltage in order to provide proper regulation. This minimum voltage differential is known as the dropout voltage of the regulator circuit. A voltage regulator circuit having a low dropout voltage has many useful applications.
Three terminal integrated circuit (IC) control devices for PNP regulators are usually designed with the intention that the base drive terminal be connected directly to the base of the discrete PNP transistor. This maximizes the voltage available for powering circuitry in the device which must use the base drive terminal as a power supply. Accordingly, the circuits generally are not designed to pull the voltage of the base drive terminal more than one volt below the regulator input voltage.
In some regulator applications, it may be desirable to use an FET as the pass transistor. However, such applications may require that the gate voltage of the FET be pulled down close to ground (e.g. to create a gate-source voltage differential of several volts). Conventional regulator control circuits are not designed to operate in this manner, as discussed above.
With a three terminal IC control circuit design (for a PNP regulator), the output current and input voltage of the regulator cannot be sensed for purposes of current limiting. This is because either type of sensing would require additional terminals. Thus, the current limit point of the IC's internal base drive current limit circuitry must be set based on the anticipated current gain of the discrete transistor, and the anticipated regulator output current, to avoid regulator operating conditions exceeding the current and power handling limits of the discrete PNP transistor.
However, protection becomes unpredictable if the user chooses a discrete PNP transistor having different current gain and power handling characteristics than those anticipated by the manufacturer. For example, the user may select a PNP transistor that cannot be safely operated at the maximum base drive current allowed by the internal current limit circuitry of the control circuit.
A similar problem arises with respect to frequency compensation. An IC regulator control circuit may be used in various application circuits having output capacitors of widely different capacitance and effective series resistance (ESR) values. However, the frequency compensation circuitry of conventional IC regulator control circuits generally provides stability only for a limited range of output capacitors.
Accordingly, it would be desirable to be able to provide a three terminal voltage regulator control circuit which could be used in a low dropout regulator circuit design in which current limiting could be adjusted for different PNP pass transistors and different applications. It would further be desirable if the control circuit could tolerate a wide range of output capacitors, and if the control circuit could provide several volts of gate-source drive voltage for an FET pass transistor in a low voltage circuit.